1. Field of the Invention
This invention relates to a power source for a liquid crystal display (LCD), and more particularly to a power source for a dot matrix LCD.
2. Description of the Related Art
In a dot matrix LCD, a plurality of row lines are disposed to cross a plurality of column lines, with intervening LCD cells. The cross points of the row and column lines constitute a dot matrix.
When a signal train for one row length, which is picture signals for one row line, is supplied to the column lines through a segment driver, one row line is selectively energized through a common driver. Thus, picture elements of one row line is displayed at one time. Row lines are successively energized to achieve display of a picture plane. The selection signal applied to the row line is called common signal and the picture signal applied to the column line is called segment signal. For example, when a liquid crystal cell is applied with a voltage above a certain value, it displays white. When the cell is applied with a voltage below the certain value, it displays black.
There is known pulse width modulation (PWM) method for displaying intermediate (gray) tones. During the selection time in which the associated row is selected, there are provided a white display period and a black display period, to display gray as an average. However, as the picture panel size becomes large, the selection time becomes short and the time constant associated with each LCD cell cannot be neglected. When the number of voltage level change is different for the cases of displaying black or white and an intermediate tone, it becomes difficult to perform a desired intermediate tone display.
There is a proposal that the common signal and the segment signal are changed their polarity once at each selection time so that the LCD cell experience the same number of polarity change irrespective of the tone of display.
There is also a kind of noise which is due to the capacitive coupling of the common electrode and the segment electrode. When the voltage of a segment electrode is changed, an induced voltage change also appears on the common electrode.
French Patent No. 2541027 (Application No. EN 8302494) discloses a compensating system which is effective for reducing the noise but requires three dummy electrodes in a cell. They are one common dummy electrode which serves as a noise sensor and two segment dummy electrodes which receive the signal supplied from the sensor after inversion and amplification. The area of the segment dummy electrodes should not be reduced less than 1/10 of the active region of the cell. Thus, this compensation cannot be said to be adapted for all the types of display.
French Patent No. 2493012 (Application No. EN 8022930) and French Patent No. 2580110 (Application No. EN 8505146) do not employ the PWM method. The gray level is provided by sequentially supplying white or black signals of different length. This drive method is appropriate when the number of gray levels is about 10 or 16. This drive method requires drivers and controllers which can act at high frequencies. Thus, it cannot be said that this system is fitted for any types of display.